In lithographic printing, a lithographic plate having a hydrophilic surface coated with a hydrophobic material forming an image is mounted on a lithographic press. Typically the plate is rotated beneath a water source to spread water across the plate, and then hydrophobic ink is applied to the plate. The hydrophobic ink does not stick on the uncoated surface of the plate because of the water extending over the uncoated surface. The hydrophobic image repels the water but attracts the ink, and thus ink is applied to the image. The inked image is then used to make lithographic copies.
Many techniques for producing lithographic plates have been developed. In one common method, plates having photosensitive coatings are exposed and developed to leave a hydrophobic image on the plate corresponding to the lithographic image to be printed. The unexposed portion of the plate remains hydrophilic. According to another technique, a transparent sheet having a special coating of graphite and a binder is placed over a plate and subjected to laser beam imaging. The laser beam causes the graphite and binder to transfer to the plate surface to create a hydrophobic image on the plate. Yet another technique includes making plates from a prepared original using master imager machines that resemble photocopiers. According to still another method, a liquid ink is sprayed onto a plate through a stencil and then the plate is heated to harden the ink.
Unfortunately, the prior methods have numerous disadvantages. Some methods require special chemicals, materials or coatings on the plate and a developing or heating step to affix the image to the plate. Other methods require expensive, single purpose equipment, expensive and often potentially harmful chemicals, or considerably operator time to make the lithographic plate. Still other methods require the operator to make an original or a stencil image first and then use the original or stencil to make the plate. However, the original or stencil must be made through other means, requiring time and additional materials. In many if not most cases, the plates are used once and discarded, thereby destroying the image. As a result, short runs are often economically impractical and many businesses cannot afford the expense associated with lithographic printing.
In U.S. Pat. No. 5,206,102 there is claimed a method of reproducing an image on a printing plate, comprising the steps of (a) providing a surface layer of hydrophilic semiconductor material on the plate; (b) applying a film of a metal plating solution on said surface layer of semiconductor material; (c) illuminating selected areas of said surface layer of semiconductor material through said film of plating solution and (d) photoelectrochemically depositing a layer of oleophilic metal plating material on said illuminated areas of said surface layer of semiconductor material to form the image: However said method needs a laser imager which is an expensive tool.